1. Field of the Invention
The present invention relates to a process of fabricating a manganese-zinc-ferrite core for deflecting yokes or transformers and coils, and such a manganese-zinc-ferrite core.
2. Description of the Background
CRT display monitors and televisions now on the market have increasingly larger screens and achieve ever-higher frequencies. Accordingly, there is a strong demand for the development of a ferrite material for deflecting yokes, which has even higher electromagnetic properties than those of conventional ferrite materials, for example, ever-higher permeability and saturation flux densities and ever-lower magnetic core losses. Magnesium-zinc-ferrite has been used as such a ferrite material for deflecting yokes. However, magnesium-zinc-ferrite places has an inherent limit on the electromagnetic properties thereof and so generates much heat due to magnetic core losses when used in the form of a core. Thus, it is often difficult to use such ferrite for a deflecting yoke core used for large screens, wide-angle deflection, and high frequencies.
Among ferrite cores known to have high permeability, high saturation flux densities and low magnetic core losses, there is a manganese-zinc-ferrite core containing 50 mol % or more of Fe.sub.2 O.sub.3. However, this core is disadvantageous in that its surface electrical resistance is low and it cannot ensure inter-winding insulation. Thus, there is a steady demand for a ferrite core having high surface electrical resistance and low magnetic core losses.
With the progression of low-profile electronic equipment, transformers and coils, too, are now required to be increasingly thin. In such applications, manganese-zinc-ferrite cores containing 50 mol % or more of Fe.sub.2 O.sub.3 and having low magnetic core losses have so far been used. For such manganese-zinc-ferrite cores containing 50 mol % or more of Fe.sub.2 O.sub.3, however, it is difficult to achieve much more thickness reductions because an insulating bobbin must be used to wind wires therearound due to their low surface electrical resistance.
For this reason, transformers or coils obtained by providing wires directly around a nickel-zinc-ferrite core of high surface electrical resistance without recourse to any insulating bobbin have so far been developed, thereby achieving much more thickness reductions. In this case, however, there is also a problem that magnetic core losses must be sacrificed due to the use of the nickel-zinc-ferrite core.
In this context, too, there is a demand for a ferrite core having high surface electrical resistance and low magnetic core losses.
While the problem with the manganese-zinc-ferrite containing 50 mol % or more of Fe.sub.2 O.sub.3 has been explained, it is understood that this problem holds more or less for manganese-zinc-ferrite containing less than 50 mol % of Fe.sub.2 O.sub.3. It is noted, however, that as the amount of Fe.sub.2 O.sub.3 decreases, the problem becomes gradually insignificant.
To increase the surface electrical resistance of the aforesaid manganese-zinc-ferrite core containing 50 mol % or more of Fe.sub.2 O.sub.3, for instance, JP-A 6-295812 describes a process for subjecting a manganese-zinc-ferrite core containing 50 mol % or more of Fe.sub.2 O.sub.3 to an oxidizing treatment, thereby forming an electrical insulating layer on the surface thereof. According to this process, it is possible to obtain a manganese-zinc-ferrite core having low magnetic core losses and high surface electrical resistance. With this magnetic core, much more thickness reductions can be achieved because wires can be wound directly around the core.
However, the aforesaid conventional process of fabricating a manganese-zinc-ferrite core containing 50 moles or more of Fe.sub.2 O.sub.3 is now found to have the following problems.
(A) An extended time is needed for the formation of the aforesaid electrical insulating layer, resulting in fabrication cost increases. For instance, the "means for solving the problem" in the above publication describes that cooling to 900.degree. C. is carried at a rate of about 75.degree. C. per hour after the completion of firing (at a firing temperature of 1,250.degree. C.). Thus, about 4.7 hours are required for cooling down to 900.degree. C., i.e., a temperature drop of 350.degree. C. At 900.degree. C. or lower, cooling is carried out at a rate of 300.degree. C. per hour and air is introduced between 700.degree. C. and 400.degree. C. to form an electrical insulating layer. Assuming that the outlet temperature of the furnace is 100.degree. C., a cooling time of about 7.5 hours is needed. When the electrical insulating layer is formed under such oxidizing conditions, it is likely to crack.
(B) In the process disclosed in the above publication, a low-oxygen-concentration atmosphere using nitrogen gas is used as the firing atmosphere. However, the introduction of nitrogen gas from the outside adds some cost to the magnesium-zinc-ferrite core currently used as a deflecting yoke core and obtained by firing without recourse to the introduction of nitrogen gas from the outside.